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If the actual size of your image is only 800px and you then view it on a 1024 res screen, then of course it's going to be stretched and distorted, particularly if it's a JPEG. The only way to prevent this is to make the physical size of the image larger than the highest screen resolution available so that at most sizes, the image is compressed, rather than expanded.
Of course, this will probably mean an unfeasibly large image, so you may have to either not worry about people with higher res screens, or rethink what you're trying to do.